At the top of the Firefox window, click on the Firefox button and then select Options.At the top of the Firefox window, click on the Tools menu and then select Options.On the menu bar, click on the Firefox menu and select Preferences....At the top of the Firefox window, click on the Edit menu and select Preferences.

Click the menu button
and choose Options.Preferences.

Select the Applications panel.

Find Portable Document Format (PDF) in the list and click on it to select it.

Click on the drop-down arrow in the Action column for the above entry and select the PDF viewer you wish to use.

Note: If you want to view PDF files in Firefox using a plugin, instead of in an external PDF reader application, select Use PDF reader name (in Firefox).

Click OK to close the Options window.Click Close to close the Preferences window.Close the Preferences window.